(i) Field of the Invention
This invention relates to components for weighing scales and more particularly to sealed hydraulic load cells. It also relates to weighing scales including such hydraulic load cells.
(ii) Description of the Prior Art
Present weighing scales, especially those for weighing vehicles in motion, generally have four or more load cells. These load cells are either of the strain gauge or of the linear variable differential transformer type. Usually, this means that one load cell is placed at each corner of the scale and the vehicle driving over the scale progressively loads and unloads the load cells as the vehicle moves across the platform. The process of sampling these multiple load cells and reconstructing the signal for final resolution to provide a readout proportional to the weight has proven difficult and costly.
Moreover, prior hydraulic load cells are generally not suitable for highway scale weighing as they have too slow a response due to the large amounts of vertical travel. In addition, some of the prior load cells are provided with diaphragms with rolled or folded edges, and the elasticity of the diaphragm results in excessive vertical travel. In addition, prior hydraulic load cells have required accurate placement of the hydraulic line between the pressure chamber and the transducer.
Among the prior patents which are directed to such load cells and such weigh scales are the following U.S. patents:
______________________________________ 1. D. F. Axelson 1,585,634 May 25, 1926 2. L. S. William 2,646,236 July 21, 1953 3. Ellis 3,082,834 4. E. W. Schellentrager 3,123,166 March 3, 1964 5. C. E. Lee 3,266,584 August 16, 1966 6. Farquhar 3,354,973 7. D. E. Smith 3,472,329 October 14, 1969 8. Schwartz 3,658,143 9. F. M. Conley 3,797,593 March 19, 1974 10. C. D. Bradley et al 3,933,212 January 20, 1976 11. K. H. Nordstrom et al 3,935,914 February 3, 1976 12. A. Goldberg 3,966,001 June 26, 1976 13. K. Von Drygalski 3,980,148 September 14, 1976 14. H. A. Wagner 3,999,621 December 28, 1976 15. M. Ohta et al 4,078,623 March 14, 1978 16. G. Dyck 4,064,955 December 27, 1977 17. P. L. Christiansson 4,102,422 July 25, 1978 18. V. S. Czyryk 4,134,467 January 16, 1979 ______________________________________
Because of their shock-proof nature, hydraulic load cells are assuming greater and greater significance in both the static and dynamic weighing field. Loads with a steep wave front where the weight might appear as suddenly as the natural frequency of single or multiple column load cells have a tendency to damage or to destroy a bridge gauge system. Also, because of the stiffness of the design of column load cells, it becomes increasingly more difficult to build in protection for sudden overloads.
In addition, Dyck, in Canadian Pat. No. 1,060,910 issued Aug. 21, 1979 disclosed the use of an hydraulic load cell including a straight diaphragm with a fluid passage leading to a transducer in such a manner that, when the transducer was removed the fluid could escape unless prevented by a valve. In that patent, the hydraulic load cell was positioned between the platform and the base structure. The load cell included a piston vertically movable within a cylinder having an upper portion and a base portion. A load on the platform transmitted force to the piston through an adjustable force-transmitting member and ball. The force on the piston increased the pressure in the pressure chamber of the load cell, which pressure was measured by a suitable transducer which communicated with the chamber by means of a passageway.
Because of the interconnection between the load cell chamber and the transducer, it became necessary, periodically, to refill and to bleed the system for absolutely air-free operation. In addition, installation was difficult due to problems of accurately lining up the interconnection line or lines.